A new wiring harness is a must.
One can either purchase a new MGB harness and modify it as needed for custom additions, or one can buy a universal wiring harness and route the wires as desired.
The cost of doing it either way is about the same, about $500 in materials, plus the time to add whatever is required, then to wrap it all up. The choice would depend upon whether one wanted to retain original components, such as the Lucas fuse boxes, or whether one wanted to upgrade to modern components. I have done both.
Modern options involve the addition of relay blocks for headlamps and other high-draw accessories, multiple fuses for additional circuits, power taps for charging or diagnosis, and the move to modern blade-type fuses, versus the old glass tubes.
Even when employing an original-type harness, I would still advocate the addition of the extra circuits and components, it just takes a bit more work and ingenuity to hide it all.
Once a high-output alternator had been fitted, and supply is no longer an issue, think of all of the nice modern additions that can be made, the majority of them invisibly, or with the subtle addition of a simple switch.
The last custom I built had, in no particular order:
Seat Heaters
Cruise control
Automatic daytime running lights, operating at 40% during the day, full on at night
Power windows
Power door locks
Alarm system, with keyless entry and remote start
Heated rear window, with automatic timer
SmartScreen intermittent wiper control
Solid-state flashers
Luxeon LED signal lights
H4 Headlamps
Stereo, with 6-CD changer, amplifier, subwoofer and component speakers.